Printed circuit boards are used to fix and electrically connect electronic components and to connect these, in the form of a module, in electronic devices. Printed circuit boards are generally composed of a plurality of alternating layers made of insulating material and conducting material, wherein the layers made of electrically conducting material, such as copper, are structured to form conductor tracks, which are connected to the contact pads of the electronic components and route these accordingly. In addition to fixing and connecting the electronic components to each other, printed circuit boards can additionally assume a static function in an electronic device given the relatively high mechanical stability of the same.
In keeping with the steadily progressing miniaturization of electronic devices, such as mobile telephones, tablet computers and the like, the increasingly smaller electronic components are not only mounted and soldered to the surface of printed circuit boards, but also integrated into the interior, the cross-section of the printed circuit boards, to an ever greater extent. A conventional method for embedding electronic components in printed circuit boards provides for the creation of clearances or holes in the printed circuit board having the size and shape of the component to be embedded so that, in a subsequent step, the components can be inserted in the respective holes and glued in place there. The disadvantage of gluing the components in the printed circuit board, however, is that the adhesive necessarily represents a different material than the material of the insulating layer in the printed circuit board. While the insulating layers in the printed circuit board are made of prepreg materials, such as FR4, which is to say resin system, adhesives for inserting the components are usually solvent-based. The bonding of components therefore automatically results in inhomogeneities, wherein the relatively strong heating of the components in the printed circuit board during operation results in the formation of cracks over time due to differing coefficients of thermal expansion of the materials that are used, which adversely affects the durability of the printed circuit board, and thus of the corresponding electronic devices.
A method was therefore provided in WO 2012/100274 A1 in which the components are not fixed in the printed circuit board by way of adhesive, but fixation is achieved by way of the resin of the insulating layers of the printed circuit board. In the production of printed circuit boards, the insulating layers, which are made of a prepreg material, are joined to the conducting layers by lamination and compression at elevated temperatures. Prior to the lamination and compression at the elevated temperature, the resin of the prepreg materials is in a non-cured state, which is referred to as the B-stage. B-stage prepregs are stored on rollers while being cooled so as to prevent premature curing of the resin. The method according to WO 2012/100274 A1 now provides for clearances for the components to be created in a combination that comprises curable prepreg material, which is to say B-stage prepreg material, for the components to be mounted in the clearances, and the clearances or openings comprising the components to be covered with further B-stage prepreg layers, and optionally with further contact layers or electrically conducting layers. This combination is subsequently compressed to yield the finished printed circuit board, in which the prepreg material is present in the cured state, the cured state being referred to as the C-stage in professional circles.
The disadvantage with this method is the circumstance that covering the clearances comprising the components with further layers is necessarily required, so that the printed circuit boards produced by way of the method according to WO 2012/100274 A1 were relatively thick, which is contrary to the idea of miniaturization.